Physics design

calculations for the W7-X

divertor scraper element

J.D. Lore1,2, J.M. Canik2, J.H. Harris2, J. Tipton3, A. Lumsdaine2

[1] Oak Ridge Associated Universities

[2] Oak Ridge National Laboratory

[3] University of Evansville

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W7-X: a high-performance, long pulse

stellarator

• Optimized for low neoclassical transport, small bootstrap current

• R= 5.5 m

• a= 0.53 m

• B= 3.0 T

• ECRH = 10MW (CW)

• ICRH = 2MW

• NBI = 4-8MW

• Superconducting coils

• Pulse length= 30 min

• Operations begin in 2015

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W7-X will use an island divertor for

power exhaust

• Island chain at edge defines last closed flux surface

• Helical X-point, qualitatively similar to poloidal divertor in tokamak

• Concept validated in W7-AS

• Restricts edge transform value allowable

– ιb ~ 1 in W7-X (5/5 islands)

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Edge transform changes as bootstrap

current evolves

• Some configurations have finite bootstrap current

• Evolves on L/R time ~ 30s

• Changes boundary transform by ~5%

– Alters island topology

– With ιbvac ~ 1, full bootstrap

current results in limiter configuration

IBS=0 kA IBS=22 kA IBS=43 kA

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Edge transform changes as bootstrap

current evolves

• Some configurations have finite bootstrap current

• Evolves on L/R time ~ 30s

• Changes boundary transform by ~5%

– Alters island topology

– With ιbvac ~ 1, full bootstrap

current results in limiter configuration

– Present plan: start with vacuum transform reduced so that island divertor is formed with full IBS

IBS=0 kA IBS=22 kA IBS=43 kA

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4

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Scraper element added to protect tile

edges during intermediate IBS

phase

• During bootstrap current build-up, field line footprints focus excessive heat flux on divertor tile edges

• New ‘scraper element’ is being designed to block field lines from reaching divertor edges in intermediate IBS

configurations

foot prints at IBS =22kA

horizontal target

vertical target

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Field line tracing is used to estimate

heat flux

• Field lines are followed in 3D including plasma contribution

– VMEC[1] for 3D equilibrium (only gives field inside LCFS)

– Extender[2] for plasma+coil fields outside LCFS

• Lines are initialized randomly along a field line that traces out a closed surface

– Lines are then diffused with a given Dm

– Example: 2000 lines, 10000 transits, Dm=1e-6 m2/m

– For 50eV electrons this corresponds to χe = 4.2 m2/s

– Sensitivity studies to be performed with respect to these parameters

• Intersections of field lines with targets used to estimate plasma fluxes

1 2 3 4 5 6 70

5

10

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-1

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X (m)

Z (

m)

[1] S.P. Hirshman, et al, Comp. Phys. Commun. 43, 143 (1986).

[ 2] M. Drevlak, et al, Nucl. Fusion, 45, 731 (2005).

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Field line tracing shows function of

divertor scraper element

• Intersections are found both with and without the scraper element

• 0kA: Load is near middle of horizontal target, no load on scraper

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foot prints at IBS =0kA

horizontal target

vertical target

foot prints at IBS =0kA

horizontal target

vertical target

scraper element

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Field line tracing shows function of

divertor scraper element

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4.5

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0 0.5 1 1.5 2 2.5

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foot prints at IBS =22kA

horizontal target

vertical target

foot prints at IBS =22kA

horizontal target

vertical target

scraper element

• Intersections are found both with and without the scraper element

• 0kA: Load is near middle of horizontal target, no load on scraper

• 22kA: Without scraper edges of target are loaded; scraper catches most of this flux

10 Managed by UT-Battellefor the U.S. Department of Energy

Field line tracing shows function of

divertor scraper element

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0 0.5 1 1.5 2 2.5

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foot prints at IBS =43kA

horizontal target

vertical target

foot prints at IBS =43kA

horizontal target

vertical target

scraper element

• Intersections are found both with and without the scraper element

• 0kA: Load is near middle of horizontal target, no load on scraper

• 22kA: Without scraper edges of target are loaded; scraper catches most of this flux

• 43kA: Footprint has moved away slightly from the target edge, scraper load reduced

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Field line density used to estimate

heat flux

• Heat flux is calculated by assuming each field line carries a fraction of the power through the LCFS

– Pline = PLCFS/Nline

• Surface is then split into area ‘bins’, with Q = (# strikes)∙Pline/Abin

• Highest scraper heat flux in current scan occurs for 22kA of bootstrap current

• Heat flux calculations act as input to engineering design

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Q (MW/m2)

IBS =22kA

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Heat flux feeds into heat transfer

calculations*

• Scraper elements will be constructed from CFC monoblocks (qualified for ITER)

– Rated for 20MW/m2 steady state, design goal for SE is 12MW/m2

• Water cooled with twisted tape piping

• Monoblocks can be arranged toroidally or poloidally (shown), with number of circuits as a design option

30mm

30mm

9mm

12mm

4mm

*Modeling performed by J. Tipton

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Initial calculations indicate peak

temperature of 976°C

• Calculations are for a conservative model of the highest heat flux in steady state

– The peak heat flux was applied over a single band with the wetted area chosen to match the total power to the scraper (400kW)

– Fluid and thermal modeling performed using ANSYS CFX

– All design criteria (pressure drop, fluid temp rise, max CFC temp) satisfied

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Future work

• Heat flux calculations

– Sensitivity studies must be performed, e.g., number of field lines, magnetic diffusivity, number of transits

– Scan magnetic configuration space to determine if other high flux scenarios exist

– Geometric studies to determine effects of misalignments of elements, gaps between tiles

• Thermal and structural modeling and design

– Modeling/design of 180°connections between monoblock channels

– Structural modeling of elements

– Changes to scraper geometry to meet constraints

• Iteration between flux calculations and engineering design

• Longer term: 3D modeling using the EMC3-Eirene [1] to investigate the effect of the scraper element on neutrals and recycling

[1] Y. Feng, et al, Plasma Phys. Controlled Fusion 44, 611 (2002)

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Extra slides

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All design criteria satisfied even for

conservative model of ‘worst case’

Variable Limit Type Result Source

Pressure Drop 14 [bar] Constraint 8.5 [bar] ± 10% Semi-Empirical

Mean Fluid Temp Rise 50 [C] Constraint 34 [C] Energy Balance

Local Fluid Temp 224 [C] Constraint N/A Assumed Non-active

Max CFC Temp ≈1200 [C] Objective 976 [C] ± 10% CFD (Grid Convergence)